1. Field of the Invention
The present invention relates generally to the field of devices for binding sheets of paper together and more particularly to an adhesive-backed binding strip for binding individual sheets or pages together in a book-like fashion.
2. Background Art
There are several well known techniques for binding pages together which do not require stitching as required in conventional book binding. For example, General Binding Corporation of Northbrook, Ill. markets a binding system under the trademark "Therm-A-Bind." The binding system includes a sheet of heavy paper stock which is folded upon itself to form the front and back covers of the bound volume. The central portion of the sheet is provided with three narrow bands of hot melt (heat-activated) adhesive. The pages to be bound are first stacked with an edge of the stack being positioned adjacent the adhesive bands. The cover is then folded around the pages and inserted in a binding machine which is provided with apparatus for melting the adhesive. The assembly is removed from the binding machine with the pages bonded to the cover sheet when the adhesive is cooled.
The primary disadvantage of the above-described binding system is that the cover sheets are an integral part of the system. Thus, there is no flexibility in the selection of cover sheets. In addition, the cost of the system is greatly increased by the inclusion of the cover sheets.
As a further example, U.S. Pat. No. 3,531,358 to Rost et al. discloses a binding apparatus which includes a strip of formable material coated with a layer of heat-activated adhesive. The individual pages to be bound are assembled together and packed so as to form a stack. The length of the strip is equal to the length of the edge of the stack to be bound with the strip width being somewhat greater than the stack thickness. The strip is positioned adjacent the stack edge and heat is applied causing the adhesive to become molten. The molten adhesive causes the edges of the sheets to be bound together upon cooling. Next, the edges of the strip are folded over the top and bottom sheets of the stack. Heat is then applied to the strip edges, thereby resulting in an adhesive bond between the strip and the top and bottom sheets (cover sheets) of the stack when the adhesive cools.
One shortcoming of the Rost et al. apparatus is that a single type of adhesive is used to perform two disparate functions. It is preferable that the adhesive adjacent the edge of the stack be of a relatively low viscosity when molten so that the adhesive is drawn up between the pages a significant distance. It is also preferable that the adhesive on the edges of the strip which bond the strip to the cover sheets be of a relatively high viscosity when molten so that the adhesive does not run out the edges of the strip onto the cover sheets. Thus, single adhesive binding apparatus of the type disclosed by Rost et al. is a compromise and is incapable of optimally performing both functions.
U.S. Pat. No. 3,847,718 to Watson discloses a dual adhesive binding strip which overcomes some of the previously-noted shortcomings of the Rost et al.-type apparatus. The Watson strip is provided with a relatively thick longitudinal central band of heat-activated adhesive which is flanked by two relatively thin longitudinal bands of adhesive flanking the central band. The central adhesive band is said to be of the low tack variety with the side bands being of the high tack variety.
Although the Watson dual adhesive strip is an improvement over the single adhesive Rost et al. device in some respects, the dual adhesive strip does possess certain disadvantages. By way of example, a ridge is sometimes formed at the two junctures of the relatively thick and thin adhesive bands which detracts from the appearance of the bound volume. The ridge is especially pronounced when the bound pages are substantially thinner than the width of the central adhesive band. Perhaps more importantly, it appears that the adhesive used in the central band of the Watson strip must possess a viscosity in the thermoplastic state which is higher than the optimum viscosity. As previously noted, it is preferable that the adhesive applied to the edges of the pages have a very low viscosity at the application temperature so that the adhesive will be drawn up between the pages. Preliminary tests indicate that if an optimum low viscosity central adhesive is used on a Watson-type strip, the adhesive sometimes has a tendency to flow over the relatively thin bands of high tack adhesive flanking the central band. The presence of the low tack adhesive on the bands of high tack adhesive greatly reduces the strength of the bond to the cover sheets.
The present invention overcomes the above-noted disadvantages of the prior art devices. The disclosed binder strip utilizes two adhesives, one optimized for bonding the edges of the pages to one another and to the substrate and the other optimized for bonding the substrate to the cover sheets. Very low viscosity adhesive can be used for the central adhesive without interfering with the bond between the substrate and the cover sheets. In addition, the bound volume is devoid of unsightly ridges, even when the thickness of the bound pages is substantially less than the width of the central adhesive band. These and other advantages of the subject invention will become apparent upon a reading of the following detailed description of the invention together with the drawings.